May 8, 2012 -- Diarrhea is a common side effect of antibiotic use, occurring in almost 1 in 3 people who take the drugs. But new research suggests that probiotics may help lower the risk of that unwanted side effect.

By affecting good bacteria, as well as bad, antibiotics can disrupt the delicate microbial balance in the intestines, but the live microorganisms marketed as probiotics can help restore this balance to reduce diarrhea risk, a new review of the research suggests.

Supported by a federal grant, researchers from the nonprofit research and analysis group RAND Corporation pooled the best available research on probiotics and antibiotic-associated diarrhea, including the most recent studies.

They found that in people taking antibiotics, those who used probiotics were 42% less likely to develop diarrhea.

The review appears this week in the Journal of the American Medical Association.

Even with the latest research, the science showing that probiotic foods and supplements promote digestive health has not yet caught up to the hype, and many questions remain about their benefits, experts tell WebMD.

"The good news is that a lot of extremely high-quality research is going on now," says gastrointestinal disease researcher Eamonn Quigley, MD, of Ireland's University College Cork, who was not involved in the review.

"Up until now, most of the noise about probiotics has been generated by marketing, but it may soon be generated by the science."

Probiotics and GI Health

Found in yogurts with live bacterial cultures, as well as in other foods and dietary supplements (a list can be found below), "probiotic" products continue to multiply on the shelves of grocery stores and vitamin and supplement retailers.

Global sales of probiotic foods and supplements reached $21 billion in 2010 and were projected to reach $31 billion by 2015, according to one market analysis.

Studies of Commercial Products Limited

Newberry says none of the studies included in the analysis examined commercially available probiotic yogurts, and very few examined commercially marketed probiotic supplements.

"In most cases these were mixtures created in the lab for the individual study," she tells WebMD.

Many types of bacteria or yeasts are considered to be probiotics, and commercially available supplements contain different combinations of these microorganisms.

"At this point the research doesn't say much about which microorganisms work best," she says.

And because dietary supplements are not regulated in the United States, buyers are on their own trying to figure out which ones to take.

"I'm afraid nothing in this review will help consumers choose which probiotic supplement to choose or which foods to eat," says David Bernstein, MD, who is chief of the division of hepatology at North Shore University Hospital in Manhasset, N.Y.

All agree that more study is needed to identify which microorganisms best benefit the gut.

"In high-risk patients -- which would include elderly people in nursing homes taking antibiotics -- it is probably not a bad idea to give a probiotic," Quigley says. "But if you ask me which one, I really couldn't tell you."

Food Sources for Probiotics

Even if there aren't recommendations on specific products, there are food sources for probiotics:

Yogurt that contains live bacteria: Not all yogurts have these. Make sure the label says "live culture," "live bacteria," or "probiotic." Buttermilk and acidophilus milk.

Cheese with live bacteria cultures: Aged cheeses such as cheddar and blue cheese are a good source, but don't cook them. Heat kills the bacteria cultures.

Kefir: a yogurt-based drink found in most major food outlets.

Miso and Tempeh: different forms of fermented soy. Miso is a paste used for seasoning and tempeh is a fermented version of soy often used as a meat substitute.

Fermented cabbage: Sauerkraut is the German version; Kimchi is the Korean style. But heavily processed products packaged in cans or jars probably don't have live bacteria. Check the label.